1. Field of the Invention
The present invention relates to electronic cameras, and, more particularly, to a camera having a reflective mirror that reflects an image-forming luminous flux to a view finder optical system or transmits the image-forming luminous flux to an image pickup device.
2. Related Art Statement
Electronic cameras have been often provided with a video display unit such as an LCD. The video display unit presents a plurality of advantages. For example, the video display unit may be used as an electronic view finder to determine the field of view and composition during picture taking, and then to check a photographed image after the picture is taken.
In such a video display unit, the LCD consumes a large amount of power, and if zooming, auto-focusing and electronic flash charging are performed at the same time, the electric load of the camera is increased, drawing a large current. Consequently, a battery cannot drive the operations, or the life of the battery is shortened. High-end electronic cameras, which are today commercialized, employ an image pickup device having a pixel array with a degree of integration as high as or higher than 100 million pixels. In contrast, the LCD has a pixel array of 200,000 pixels or so, and the resolution of the LCD is unable to match that of the image pickup device. As a result, the high-resolution image pickup device is not fully exploited, for example, a correct focus cannot be detected with the LCD display in a strict sense.
An optical system, of a quick-return mirror and a penta prism, used in conventional silver-film based cameras may be incorporated in an electronic camera to give a parallax-free and sharp image of a subject for checking.
The means of using the quick-return mirror prevents the luminous flux from reaching the image pickup device during the observation through the view finder optical system. For this reason, the image pickup device remains unable to directly perform auto-focusing (AF), auto-exposure control (AE), and auto-white balancing (AWB). The electronic camera thus suffers a time delay because the image pickup device has to take these process steps after the mirror is flipped up.
To cope with this problem, an optical system for a single-lens reflex camera has been proposed. In such a system, a transflector is arranged between an imaging optical system and an image pickup device. The transflector splits an image-forming luminous flux, guiding part of the luminous flux to the image pickup device and the remaining luminous flux to a view finder optical system
Japanese Examined Patent Publication No. 6-29925 discloses a view finder optical system. In the view finder optical system, a prism body having a transflective surface is arranged downstream of the imaging system. Part of the image-forming luminous flux, transmitted through the imaging system, is reflected from the transflective surface and is then totally reflected from the surface of the prism body facing the subject, exits from the prism body, and is focused as an image of the subject. The image of the subject is thus observed in the view finder optical system. Let .theta. represent the smaller one of the angles between the transflective surface and the optical axis of the imaging system, and the view finder optical system satisfies the condition of 23.degree.&lt;.theta.&lt;38.degree..
With such a view finder optical system incorporated in an electronic camera, AF, AE and AWB operations are carried out during the observation using the view finder optical system.
However, the above means of using the transflective surface halves the amount of light incident on the image pickup device and presents a darker image observed through the view finder optical system, because the luminous flux is split. Furthermore, the light ray is fed back from the view finder optical system to the image pickup device, adversely affecting the accuracies of AF, AE, AWB and the photographed picture.